In general, phosphatidylcholines contain as a constituent fatty acid thereof an unsaturated fatty acid such as oleic acid, linoleic acid, linolenic acid, docosahexaenic acid or arachidonic acid, so that they have the property of being readily oxidized. In particular, when phosphatidylcholines are purified to high purity, they are separated from neutral lipids or proteins which have existed as impurities and acted just like protective layers, and directly exposed to oxygen in the air, so that they are oxidized more easily.
For this reason, there has been made an effort to prevent phosphatidylcholines from being oxidized, e.g., during storage. Generally, they are stored in a dark and cool place or in an inert gas atmosphere; an antioxidant such as vitamin E is added to them; or an unsaturated bond contained in them is converted into a saturated bond by hydrogenation.
Phosphatidylcholines which are purified to high purity are often used for cosmetics, drugs or the like. In such cases, they are usually utilized as liposomes, emulsifying agents or the like. It has been known that, in such cases, the smaller is the amount of impurities such as inorganic salts and free amino acids, the better are the products obtained. (See, for example, Japanese Laid-Open Patent Publication No. 3115/1989.)
However, although storage in a dark and cool place or in an inert gas atmosphere is effective for phosphatidylcholines prepared, it is impractical and almost impossible for those products such as cosmetics and drugs which are produced by using the phosphatidylcholines.
Further, the addition of an antioxidant to phosphatidylcholines is unfavorable when their use in cosmetics, drugs, foods or the like is taken into consideration. There is also a case where an antioxidant cannot be added.
Furthermore, in order to conduct a hydrogenation treatment, it is necessary to additionally provide a step for this purpose.
In addition, the stability of liposomes or emulsified products obtained by using phosphatidylcholines is reduced when the phosphatidylcholines are oxidized. Moreover, the phosphatidylcholine oxides also bring about the problem of toxicity for the body or the like.
It has thus been general rules that purified phosphatidylcholines should be stored in a dark and cool place immediately after the preparation thereof and used for producing desired products as soon as possible, and that the products obtained should also be consumed in a short period of time.
Under these circumstances, it is now acknowledged that the development of phosphatidylcholines which are per se excellent in oxidation stability is strongly required.
Therefore, an object of the present invention is to provide phosphatidylcholines excellent in oxidation stability and thus can meet the above requirement.